I write an aplication using 2 UARTS and timer. The UART0 and UART1 are full interrupt controlled. I use UART0 to send and get commands to target device. UART1 is used to communicate with an external device over RS485. UART1 send a comand and receive the response ( all via interrupt handlers ). After receiving it send a new command and so on.
The behaviour is the following:
The application run ca. 20sec. Than the EIC->IER bit of UART1 is reset. The rest of the aplication works well (inclusive the timer interrupt) but I don't get interrupts from UART1. Sometimes UART0 isn't working too. When I enable the EIC->IER bit of UART1 again UART1 doesn't work again. The regiter values of UART1 seems OK.

Do you have an idea why the EIC->IER bit is reset?

Some aditional informations:
In normal working state the EIC register is 0x1C0C01. When UART1 stops the EIC register is 0x1C0801. The interrupt pending register is 0x180001 when UART1 stops
I check the IRQ, and C-Stack -> OK
I changed the interrupt priority -> no chaning
I removed all code wich disable flags in EIC->IER -> no changing